The principal thrust of the projects during the past year has been to establish a reliable method to measure transcapillary exchange of bone seeking mineral ions. In a series of studies in dogs, blood flow (F) was estimated by iodoantipyrine (I-Ap), Sr 85 clearance (C) and Sr 85 extraction (E) were determined by concurrent indicator technics. These experiments showed ESr is nearly independent of F, so that Sr 85 clearance (C) does provide a simple reliable index of flow. This data has led to a project to evaluate effects of different forms of fracture fixation (intramedullary rods and plates) on bone blood flow and rate of healing at the fracture site judged by combined microradiograph and tetracycline measurement of bone formation rates. Studies now near completion utilizing concurrent indicator technics (Sr85, C14-labeled sucrose and Cr51-labeled albumin) indicate that free diffusion is the principal mechanism for movement of Sr85 across the bone capillary wall. The same technic was employed in studying transcapillary movement in dogs made hyperparathyroid. These studies conclusively showed no difference in permeability of Sr85 in hyperparathyroid dogs from control dogs.